The easiest way to install MightyPHP is to use Composer:
composer require mightyphp/mightyphp
Or you can start a project immediately with:
composer create-project mightyphp/mightyphp project-name-or-path
After that, just run
php mighty startfrom within the project root folder to start up the development server.
At this time and age, it is quite tedious to work without a good package manager. Get composer at: https://getcomposer.org/